Numerical Analysis Of Stability Of Submarine Pipeline Under Wave Loading In Chengdao Oil Field

Submarine pipeline is the important part of ocean oil and gas transportation and reposition system. It is the lifeline of ocean oil field. The stability of submarine pipeline is a key question for pipelines engineering. CoMPared with the pipeline on land, submarine pipeline run in a harder environment, the probability of destability is much higher than former. The pipeline in service is easy to lose destability under wave, current and tide load. At the present time, few investigation about the stability of submarine pipeline with given hydrologic and geologic file is reported. For pipeline laid on seabed, the induced wave force is calculated with Morison model, neglect the wake effect to the velocity of water around pipeline. And to the buried pipeline, the induced wave force is calculated mostly based on the linear wave theory, and the contact effect between pipeline and soil is neglected. The analysis about pipeline is mostly about the wave induced pore pressure stress and strain ect. in the soil around pipeline, few study about the behavior of pipeline in the liquefied soil is reported. Contraposing these shortages, a more reasonable numerical model of pipeline/soil is developed, and the stability of submarine pipeline in Chengdao oil field is analyzed based on the model. The work of this dissertation mainly includes:1.Collecting and analyzing file about 138 bores, classing Chengdao oil field 3 different zones, and hydrologic and geologic characteristic of each zone is dissertated. This work has an important tpracticality for oil reconnoitring and transporting in this area. Based on Biot theory and hydrologic,geologic file of Chengdao oil field.2.Based on Biot theory and hydrologic,geologic file of Chengdao oil field, a numerical model is developed for analyzing the stability of submarine pipeline. The ii WakeⅡmodel is imported into ABAQUS by developing a subroutine‐DLOAD, this make the numerical model more reasonable coMPared with previous models. The reliability is testified by coMParing with the relevant research. The main factors of affecting the stability of submarine pipeline are analyzed, and the result shows: effect of Wake has disadvantage for pipeline stability, weight of per length unit pipeline is a important factor affect the stability of pipeline. Based on the result of numerical model, a set of curve for checking the stability of pipeline in Chengdao oil field is discussed.3. A numerical model of pipeline/soil under the nonlinear wave and random wave loading is developed. In the dissertation, the wave loading is calculated based on nonlinear wave theory and random wave theory, and so, the nonlinear effect and randomicity of wave is considered. Besides, the contact effect between pipeline and soil is considered. Compared with previous models, the numerical model in this dissertation is more reasonable.Based on this model, the response of pipeline/soil under the wave loading is analyzed,and the result showes that the nonlinear effect of wave and contact effect between pipeline and soil are can not neglected.4. Based on the finite strain theory, a subroutine‐UEL for ABAQUS is developed , and the residual pore pressure under wave load is computed, and stability of soil around pipeline and seabed is analyzed. Regarding the liquefaction soil as model, a subroutine‐UMAT for ABAQUS is developed , and the behavior of pipeline in liquefaction soil analysis qualitatively. The stability of buried pipeline is analyzed with numerical model, and the numerical result shows that: the coefficient of permeability affect the cumulation ratio of pore pressure remarkablely; The coefficient of permeability is greater, the pore pressure cumulates more slowly, and vice versa. The depth of liquefaction increases with the wave load's action time, and finally tend to a constant.To the 3 different geologic areas, the depth of liquefaction has the followed relation:Ⅰ<Ⅱ<Ⅲ.The viscidity coefficient of liquefaction soil affect the velocity of pipeline only, can not affect the movement trend. With the numerical models, a formula for checking the stability of pipeline is discussed. The main innovation points in this dissertation include:According to the hydrologic,geologic file of Chengdao oil field, the more reasonable hydrodynamic model is adopted to compute the load of pipeline laid on seabed.A numerical model of pipeline laid on seabed and soil interaction in Chengdao oil field is developed,a set of curve for checking the stability of pipeline in Chengdao oil field is discussed.Moreover,a numerical model of embeded pipeline/soil interaction under complex wave laod is developed, the behavior of pipeline in the liquefied soil is simulated base on finite distortion theory, finaly the influence factors are analysed and a set of conclusions are put forward.